Characterization of nontyphoidal Salmonella strains from a tertiary hospital in China: serotype diversity, multidrug resistance, and genetic insights

Abstract

Objective: Nontyphoidal Salmonella is a significant public health concern due to its ability to cause foodborne illnesses worldwide. This study aims to characterize the nontyphoidal Salmonella strains isolated from patients in China.

Methods: A total of 19 nontyphoidal Salmonella strains were characterized through serovar identification, antimicrobial susceptibility testing (AST), biofilm formation assessment. Genetic relatedness was determined using pulsed-field gel electrophoresis (PFGE). WGS was employed to decipher the resistance mechanism and to contextualize the S. serovar Mbandaka strains among previously sequenced isolates in China. The biofilm associated mrkA gene was examined by PCR.

Results: The predominant serovar identified was S. Enteritidis, followed by S. Mbandaka, S. Thompson, S. Livingston, S. Alachua, and S. Infantis. PFGE analysis indicated a notable genetic similarity among the S. Mbandaka isolates. Phylogenetic analysis suggested that these strains were likely derived from a single source that had persisted in China for over five years. One multidrug resistance (MDR) S. Enteritidis isolate carried a highly transferable IncB/O/K/Z plasmid with bla _CTX-M-15. One S. Thompson strain, harboring the mrkABCDF operon in an IncX1 plasmid, isolated from cutaneous lesions, demonstrated robust biofilm formation. However, no mrkABCDF loci were detected in other strains.

Conclusion: Our study emphasizes the importance of persisted surveillance and prompt response to Salmonella infections to protect public health. The dissemination of bla _CTX-M-15-harboring IncB/O/K/Z plasmid and the spread of virulent mrkABCDF operon among Salmonella in China and other global regions warrant close monitoring.

Keywords: antibiotic resistance; biofilm; mrkABCDF operon; nontyphoidal Salmonella; phylogenetic analysis.

Figure 1

Time frame of stays in hospitalization for the 19 patients involved in NTS infection. Horizontal bars represent the duration of hospitalization for each patient, with distinct colors indicating the respective ward departments. Circles represent the time of isolation for each patient with colored circles representing the specimen source. Colored stars represent different serovars.

Figure 2

Dendrogram of the pulsed-field gel electrophoresis (PFGE) patterns of NTS isolates collected from this study. M, PFGE marker strain (S. Braenderup H9812).

Figure 3

Phylogenetic relationships within S. Mbandaka strain JNQH948 and publicly available sequences from EnteroBase and a Chinese study. Phylogeny (left) showing the relationships. The strain source, collection year, location and specimen types were shown for each isolate. Subclade containing the JNQH948 and the closely related S. Mbandaka strain were indicated in light green shade.

Figure 4

A circle map of the bla _CTX-M-15-bearing IncB/O/K/Z type plasmid pJNQH940-1 in strain JNQH940. Open reading frames (ORFs) of pJNQH940-1 are shown as the outermost ring, with plasmid replicons, transconjugation elements (T4CP, T4SS, oriT, relaxase), type IV pilus system and antimicrobial resistance genes highlighted.

Figure 5

Biofilm formation of NTS strains. Data are represented as mean ± standard deviation from at least three independent experiments.

Figure 6

Genetic comparison of mrkABCDF harboring plasmid pJNQH950-2 and two closely related to plasmid pCP8-3-IncX1 (NZ_CP053740.1), pOLA52 (NC_010378.1). mrkABCDF transposon (Tn6011) are shown as the outermost ring. The genes are exhibited in arrows with different colors to note on their functional classes. Shades indicate shared regions of homology. The plasmid accession numbers are listed below the plasmid names.

Figure 7

Distance tree of plasmids harboring mrkABCDF operon in reference NCBI database. Phylogeny (left) showing the relationships between 195 mrkABCDF operons harboring plasmids. The presence of mrkABCDF virulence loci, plasmid conjugation elements (oriT, T4SS, T4CP, relaxase), top five plasmid types, host strain properties (collection year, country, source), as well as the plasmid size and number of resistance genes were illustrated for each isolate. Grey tiles indicate gene’s absence. The isolates’ tip points are colored by the host species.